In this Chapter we discuss models involving two lives. Actuarial present values for basic benefits are derived by applying the concepts and techniques developed in Chapter 3 through 5. Models built on the assumption that two future life-time random variables are independent constitute most of the chapter. Section 9.6 introduces special models in which the two future lifetime random variables are dependent. Annual benefit premiums, reserves, and models involving three or more lives are covered in Chapter 18.